This invention relates to digital printing and more particularly to printing using devices which eject ink onto the printed substrate. However, the invention is not limited to ink ejection devices and is also applicable to laser, light emitting diode printers and to digital photocopiers.
In ink ejection devices a printhead has an array of nozzles through which ink is selectively ejected onto the substrate as the substrate moves relative to the printhead. The printhead may print by scanning across the substrate to print horizontal bands or, if it is a full page width printhead, it may pass along the length of the page. A blocked nozzle will result in multiple horizontal blank lines, in the case of a scanning type printhead, or a blank vertical line in the case of a page width printhead. Such blank lines are undesirable since they detract from the printed result.
The present invention provides a method of modifying the printing of an image so as to reduce or effectively eliminate the visual effect of one or more such blocked nozzles apparent to the eye of an observer in normal use. However, the invention is applicable to other forms of printing where a device, whether passive or active, is repeatedly used to produce dots of ink or the like on a substrate. The invention has potential application to laser and LED type printers and photocopiers where a fault in the imaging drum or light source can result in repeated faults in the image produced. As used above and throughout the description and claims the term image is to be understood to have a broad meaning and includes anything printed, such as text and line drawings.
In one broad form the invention provides a method of modifying an image to be printed by a digital printing device to compensate for failure of the device to print ink correctly at one or more specific locations, the method including the steps of identifying said specific location or locations and for at least one specific location shifting the printed position of the dot intended for the specific location in the image to a shifted location not already used by another dot of the image.
In another broad form the invention also provides a printer having a row of devices which cause rows of dots to be deposited onto a substrate and means to move the substrate relative to said row of devices in a direction generally perpendicular to said row of dots, said printer including:
a) means to determine if one or more of said devices is not operating correctly;
b) control means to analyse images to be printed and to identify when a dot of ink should be printed by activation of the failed device and to shift the position of the dot in the printed image such that the dot is printed by activation of one of the devices on either side of the failed device.
The ink dots originally intended to be printed by the defective device may be shifted transversely longitudinally or both transversely and longitudinally. Preferably the ink is shifted to a location immediately adjacent to the original location.
The defective device will result in a defect line or lines in the image printed and preferably the ink is shifted to lie on alternate sides of the line or lines. Preferably, the ink is only shifted side-ways relative to the line, but if no location is available in the same row, the ink may be shifted both longitudinally and transversely and longitudinally along the line. The ink may be shifted to a row before or a row after the original row location.